This invention relates to starting of high intensity discharge lamps and, more particularly, to a new and improved light source wherein a spiral line pulse generator is used to start a high intensity discharge metal halide lamp.
Conventional high intensity discharge metal halide lamps include two main electrodes at opposite ends of a discharge tube and an auxiliary starting electrode associated with one of the main electrodes. A starting circuit applies a high voltage between the main electrodes of the lamp and, simultaneously, between the starting electrode and its associated main electrode. A discharge is initiated between the starting electrode and the main electrode by the starting circuit and then transfers to provide a discharge between the two main electrodes. After a high intensity discharge is formed within the discharge tube, the voltage between the electrodes drops and the starting circuit is no longer operative.
While the starting electrode in metal halide lamps provides generally satisfactory operation, it has certain disadvantages. The complexity and cost of manufacturing the lamp are increased when the starting electrode is used. In addition, the lamp seal in the region of the starting electrode is adversely affected by an electrolysis process when a potential difference exists between the starting electrode and the main electrode. The degradation of the seal can eventually lead to lamp failure. It is known that this problem can be alleviated by connecting a thermal switch, which closes after starting of the lamp, between the main electrode and the starting electrode. However, the thermal switch adds to the overall cost and complexity of the lamp assembly. It is, therefore, desirable to provide a starting arrangement for metal halide lamps wherein the starting electrode can be eliminated.
The spiral line pulse generator, disclosed by R. A. Fitch et al. in U.S. Pat. No. 3,289,015, issued Nov. 29, 1966, is a device capable of storing electrical energy and, upon momentary short circuiting of a pair of terminals, of providing a high amplitude pulse. The spiral line pulse generator can, when properly utilized, provide the dual functions of storage and voltage multiplication. The spiral line pulse generator is a transient field reversal device which provides a roughly triangular pulse. Its peak voltage is a multiple of the initial charging voltage. The use of a spiral line pulse generator to start high pressure sodium lamps is disclosed in U.S. Pat. No. 4,325,004 No. 193,787, filed Oct. 2, 1980 and assigned to the assignee of the present application. The output of the spiral line pulse generator is coupled to a conductor, or starting aid, located in close proximity to an outer surface of the discharge tube. In the case of metal halide lamps, it has been found undesirable to locate conductors in close proximity to the central portion of the discharge tube, thereby ruling out the use of a starting aid to assist in initiating discharge.